As described in my earlier filed '137 application, precision fluid flow control devices, such as those used in fuel supply units for aerospace systems and oxygen/air metering units employed in hospitals, as non-limiting examples, typically incorporate a solenoid-operated valve for controlling fluid flow substantially proportional to the current applied to the solenoid. It is also desirable that hysteresis in the flow rate versus control current characteristic (which creates an undesirable dead band in the operation of the valve) be maintained within some minimum value.
A standard practice for reducing hysteresis has been to physically support the solenoid's moveable armature within the bore of its surrounding drive coil by means of low friction bearings, such as Teflon rings. However, even with the use of such a low friction material, there is still significant `dead band` current (e.g. on the order of forty-five milliamps), which limits the operational precision of the valve.
One proposal to deal with this physical contact-created hysteresis problem is to remove the armature support mechanism from within the bore of the solenoid coil (where the unwanted friction of the armature support bearings is encountered) to an end portion of the coil, and to support the armature for movement within the bore by means of a spring mechanism located outside of the solenoid coil.
An example of such a valve configuration is described in the U.S. Pat. No. 4,463,332, to Everett, issued Jul. 31, 1984. Pursuant to this patented design, the valve is attached to one end of an armature assembly supported for axial movement within the cylindrical bore of the solenoid coil and having a permanent ring magnet surrounding the solenoid. One end of the solenoid contains a ring and spring armature support assembly, located substantially outside the (high flux density) solenoid bore, and whose position can be changed, so as to adjust the axial magnetic flux gap within the bore and thereby the force applied to the valve.
Disadvantageously, however, this type of support structure requires a magnetic flux booster component which, in the patented design, is a permanent magnet. Namely, even though the objective of the Everett design is to adjust magnetic permeance and maintain linearity, the overall solenoid structure and individual parts of the solenoid, particularly the ring spring armature assembly (which itself is a complicated brazed part), and the use of a permanent booster magnet, are complex and not easily manufacturable with low cost machining and assembly techniques, resulting in a high price tag per unit.
In another prior art configuration, described in the U.S. Pat. No. 4,635,683, to Nielsen, the movable armature is placed outside the bore by means of a plurality of spiral spring-shaped bearings adjacent to opposite ends of the solenoid structure. Unfortunately, this structure is costly to manufacture, as it not only places a complicated and movable return spring structure in the interior of the solenoid bore, but requires precision attachment of the spring bearings at multiple spaced apart locations of the housing adjacent to opposite ends of the solenoid.
The linear motion proportional solenoid assembly described in my U.S. Pat. No. 4,954,799 entitled: "Proportional Electropneumatic Solenoid-Controlled Valve," improves on the above designs by using a pair of thin, highly flexible annular cantilever-configured suspension springs, to support a moveable armature within the bore of solenoid, such that the moveable armature is intimately coupled with its generated electromagnetic field (thereby eliminating the need for a permanent magnet as in the Everett design, referenced above).
In order to make the force imparted to the movable armature substantially constant, irrespective of the magnitude of an axial air gap between the armature and an adjacent magnetic pole piece, my patented structure places an auxiliary cylindrical pole piece region adjacent to the axial air gap. This auxiliary cylindrical pole piece region has a varying thickness in the axial direction, which serves to `shunt` a portion of the magnetic flux that normally passes across the axial gap between the armature assembly and the pole piece element to a path of low reluctance. By shunting the flux away from what would otherwise be a high reluctance axial path through a low reluctance path, the auxiliary pole piece region effectively `linearizes` the force vs. air gap characteristic over a prescribed range.
The proportional solenoid structure described in my earlier filed '137 application, and diagrammatically illustrated in FIGS. 1 and 2, reduces the structural and manufacturing complexity of the implementation of my previously patented structure by locating a moveable, magnetic armature 10 adjacent to one end of a fixed pole piece 12 that protrudes outside a solenoid coil bore 14, and configuring this moveable armature 10 to provide two, relatively low reluctance magnetic flux paths 21 and 22.
The first flux path 21 is a radially directed flux path through an annular air gap 31 at one end 16 of fixed magnetic pole piece 12. This annular shaped air gap results from the fact that the radially projecting, tapered rim portion 34 of the movable armature 10 has an interior diameter that is only slightly larger than the diameter of the end 16 of the fixed magnetic pole piece 12 protruding beyond the one end of the solenoid bore 14. This allows for relative axial translation between the movable armature 10 and the fixed magnetic pole piece 12 as moveable armature 10 is axially translated by energizing a solenoid coil 11. Since annular air gap 31 is very short, fixed radial distance, the magnetic flux path between the end 16 of the fixed pole piece 12 and armature 10 is a substantially constant, low magnetic reluctance radial path.
The second flux path 22 is also essentially a radial flux path through a variable geometry annular air gap 32 between a radially projecting, tapered rim portion 34 of moveable armature 10 and an inwardly projecting tapered portion 36 of the solenoid assembly housing 30. Because the thickness of each of the mutually opposing surfaces of the tapered rim portion 34 of the moveable armature 10 and the inwardly projecting rim portion 36 of the housing 30 is tapered to an annular wedge shape at the variable geometry air gap therebetween, the magnetic field characteristic between the armature and the housing becomes saturated at each of mutually adjacent tapered rim portions 34 and 36.
Consequently, the magnetic flux through the armature is principally confined in the radial direction, by-passing the substantial reluctance path along an axial air gap 23 between the moveable armature 10 and the lower end 16 of the fixed pole piece 12. This causes the force imparted by the solenoid on movable armature 10 to vary in proportion to the applied current, so that displacement of moveable armature 10 against the bias of an axial compression spring 40 varies in proportion to solenoid current.
When employed in a fluid flow application, the proportional solenoid assembly of my '137 application may include an armature retainer 25 threaded into the moveable armature 10, and mechanically coupled with a valve poppet 55 of a valve unit 50. The armature retainer 25 and the movable armature 10 capture interior radius portions of a pair of spiral suspension springs 41 and 42, mutually spaced apart by a spacer 44 of non-magnetic material, and supporting the axially moveable armature 10 outside solenoid bore 14. The valve poppet 55 engages a valve seat 56 and thus controls the flow of fluid between fluid input port 51 and a fluid exit port 52 of the valve unit 50, with displacement of the movable armature 10 along the solenoid assembly axis A being proportional to solenoid current.
Now although the proportional solenoid structure described in my '137 application has been found to operate extremely well in relatively small and larger sized hardware configurations, for very small (e.g., micro-valve) applications and using reasonable priced industry standard materials, I have observed that distortion of one or more components of the assembly may occur, as the parts--especially the moveable armature's support springs 41 and 42--become very small and dimensionally thin.
In particular, I have observed that, for very small dimension applications, what would otherwise be a negligible axial magnetic flux component accompanying the dominant radial flux component bridging the variable geometry radial air gap 32 between the saturated tapered rim portion 34 of the moveable armature 10 and the inwardly projecting tapered portion 36 of the solenoid assembly housing 30 becomes significant. By significant is meant that, absent the use of relatively expensive, mechanically robust (composite or metallurgically exotic) materials, such as titanium alloys and the like, the small amount of non-radially directed magnetic flux in the variable geometry air gap 32 is sufficient to overcome the mechanical rigidity of the material (e.g., brass) of the armature support springs 41 and 42, and causing the springs to warp or twist from their intended shape, and thereby deviate from their normal axial cantilever flexing along axis A.
This unwanted distortion of the armature support springs is particularly likely where there are non-trivial departures from dimensional tolerances in the manufacturing of the parts of the solenoid assembly. Because of the variable geometry gap inherently tends to provide some degree of play between the armature and the housing, distortion of the armature support springs can cause an unbalanced physical engagement of the tapered rim portion of the moveable armature with the inwardly projecting tapered portion of the housing, thereby impairing or preventing proper operation of the proportional solenoid assembly.